"Upgrade Complete" is an interesting take on flash games: You basically buy parts of the game using in-game cash. Want to load it? You pay $200 for the preloader, then you pay for a game menu, then if you want the game to look.. well... not crappy... you pay for that too. Pretty much everything in the game can be upgraded.

Obviously you're not using real money here, and you earn game money via the actual 'game' itself, which is a cheap space invaders knock off, but it's plenty fun and there's lots of positive reviews. Addictive? I'm not sure, but it'll kill some of your Friday while the boss isn't looking.